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Anatomy of the Furnace Thermostat

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What is a Thermostat and How Does it Work?
Honeywell T87 Mercury Switch Thermostat

Honeywell T87 Mercury Switch Thermostat

© Honeywell 2006

Home Furnace Thermostat
The furnace thermostat is the nerve center of your home heating system. It's the main control point that determines when and how much heat will be produced by the furnace. But for all its mystery, the thermostat is actually a very simple device. In fact it's simply a temperature sensitive switch.

There are two main categories of thermostats depending on the type of heating system, low voltage and line voltage.

This tutorial covers the low voltage thermostat often made by Honeywell, White-Rodgers, Lux, Robertshaw and others. It runs off of a transformer reducing the incoming line voltage from 120 volts down to between 12 and 24 volts, depending on your furnace.

Within this low voltage thermostat category, there are four major types. They include:

  • Mercury Contact (older style)
  • Mechanical Contact
  • Digital
  • Electronic Programmable (see also IP Thermostat)

    Another type of thermostat, the line voltage thermostat, is typically hooked up to a 120 volt or 240 volt electric resistance heating system. This thermostats work similar to a light switch dimmer, varying the amount of electricity to the resistance heater. Line voltage thermostats are not the subject of this tutorial.

    Let's take a look at the anatomy of each low voltage thermostat type, starting with the older style mercury contact thermostat often represented by the iconic T87 Honeywell in the photo above.

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